Abstract. The sauropterygian clade Plesiosauria arose in the Late Triassic and survived to the very end of the Cretaceous. A long, flexible neck with over 35 cervicals (the highest number of cervicals in any tetrapod clade) is a synapomorphy of Pistosauroidea, the clade that contains Plesiosauria. Basal plesiosaurians retain this very long neck but greatly reduce neck flexibility. In addition, plesiosaurian cervicals have large, paired, and highly symmetrical foramina on the ventral side of the centrum, traditionally termed subcentral foramina, and on the floor of the neural canal. We found that these dorsal and the ventral foramina are connected by a canal that extends across the center of ossification of the vertebral centrum. We posit that these foramina are not for nutrient transfer to the vertebral centrum but that they are the osteological correlates of a highly paedomorphic vascular system in the neck of plesiosaurs. This is the retention of intersegmental arteries within the vertebral centrum that are usually obliterated during sclerotome re-segmentation in early embryonic development. The foramina and canals are a rare osteological correlate of the non-cranial vascular (arterial) system in fossil reptiles. The adaptive value of the retention of the intersegmental arteries may be improved oxygen transport during deep diving and thermoregulation. These features may have been important in the global dispersal of plesiosaurians.

Plesiosaurians retain a very long neck but greatly reduce neck flexibility, and the cervicals have large, paired, and highly symmetrical foramina on the ventral side of the centrum, traditionally termed subcentral foramina, and on the floor of the neural canal. We found that these dorsal and the ventral foramina are connected by a canal. The foramen are not for nutrient transfer; they are the osteological correlates of a highly paedomorphic vascular system in the neck of plesiosaurs.

Plesiosaurians retain a very long neck but greatly reduce neck flexibility, and the cervicals...